Gallium arsenide phosphide camera tube target having a semi-insulating layer on the scanned surface

ABSTRACT

A continuous barrier single crystal GaAs1 xPx vidicon target is provided with a thin semi-insulating layer of antimony trisulfide on its scanned side to prevent higher energy beam electrons from traveling through the barrier to a signal plate as dark current.

United States Patent [72] Inventors Ralph E. Simon Trenton;

Robert L. Rodgers, Pennington, both, NJ. 754,850

Aug. 23, 1968 June 15, 1971 RCA Corporation [21 Appl. No. [22] Filed [45 1 Patented [7 3] Assignee [54] GALLIUM ARSENIDE PHOSPHIDE CAMERA TUBE TARGET HAVING A SEMI-INSULATING LAYER ON THE SCANNED SURFACE 3,458,782 7/1969 Buck et a1 313/65 X 3,403,278 9/1968 Kahng et al.... 313/65 3,474,285 10/1969 Goetzberger 313/65 X OTHER REFERENCES ELECTRONICS March 4, 1968 p. 109 Article Lighting Up In a Group" by L. A. Murray, S. Caplan & R. Klein. (complete article pp. 104- 110) SCIENTIFIC AMERICAN May 1967 p. 110 Article Light Emitting Semiconductors by Frederick Fv Morehead, Jr. (complete article pp. 108- 122) Primary Examiner--Roy Lake Assistant Examiner-V. Lafranchi Attorney-Glenn I-I. Bruestle ABSTRACT: A continuous barrier single crystal GaAs P vidicon target is provided with a thin semi-insulating layer of antimony trisulfide on its scanned side to prevent higher energy beam electrons from traveling through the barrier to a signal plate as dark current.

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GALLIUM ARSENIDE PHOSPHIDE CAMERA TUBE TARGET HAVING A SEMI-INSULATING LAYER ON THE SCANNED SURFACE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to targets for camera tubes andparticularly concerns a target having a semi-insulating layer on its scanned side and an electrical barrier region.

2. Description of the Prior Art One type ofcamera tube which is at present widely used and readily available commercially is the vidicon. Its basic structure is well known to those skilled in the art and usually comprises an elongated glass envelope having a transparent faceplate at one end. Inside the tube envelope is an electron gun device for producing a flne'electron beam. With the aid of magnetic or electrostatic deflection means, such as magnetic coils disposed just outside the envelope, this beam can be made to scan a target area on the inside faceplate surface.

The inside faceplate surface is coated first with a transparent, electrically conductive material, such as tin oxide, to form a signal plate. The signal plate is then coated with a photoconductive substance to form a target.

When the vidicon is in operation, the electron beam continually scans a well-defined area, or raster, of the target and tends to charge it uniformly to an equilibrium potential each time it scans the entire raster, or each frame time. The signal plate is impressed with a voltage a few volts above equilibrium potential, and this results in a continual dark current of elec trons from the rastersurface of the higher voltage signal plate when the photoconductor is not exposed to light. A light pattern incident on the photoconductor greatly increases current in the lighted areas. Areas on the raster which have lost charge through such current during the course ofa frame time are in the next scanning frame brought abruptly back to equilibrium potential by the beam. The sudden change in voltage of those target areas that are returned to equilibrium potential results in a signal at the signal plate. This signal is the video output of the tube.

The dark current of a vidicon target can be decreased by providing an electrical barrier between the scanned side of the target and the signal plate. Such a barrier may be a Shottky barrier or a PN junction, for example. The barrier in effect gives the target a higher dark resistance than it would have without the barrier.

An undesirable aspect of barrier-type targets, however, is that their fabrication is relatively expensive. The forming of PN junctions on the scanned side of the target usually involves a somewhat complex type of diffusion process or vapor phase growth, both of which are relatively slow processes requiring complex equipment. Formation of a Shottky barrier target usually requires application of very precise masking techniques, and those masking techniques may impose a limitation on the resolution attainable by the target.

SUMMARY OF THE INVENTION A barrier region target for a vidicon-type camera tube is provided with a thin semi-insulating layer on its scanned side. The target comprises a target crystal wafer of photoconductive semiconducting material. The wafer has a slightly N conductivity-type region in its bulk and a p conductivity-type region near and including at least a portion of one major surface of the wafer. The semi-insulating layer acts to thermalize electrons impinging on it from the scanning means and to prevent those electrons from traveling through the wafer to the signal electrode as dark current.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a sectional view ofa vidicon-type camera tube having therein a target according to a preferred embodiment of the invention; and

FIG. 2 shows a fragmentary sectional view of the charge storage target in the tube of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT General Structure In the preferred embodiment of the invention shown in FIG. 1, a conventional vidicon-type camera tube 10, having an elongated envelope 12 with a transparent faceplate 14 at one end and electron beam forming and scanning means 16 disposed inside the envelope [2, is provided with a thin semiconducting target I8 attached to the inside surface of the faceplate l4. Scanning of the electron beam may also be effected by magnetic coils (not shown) situated outside the envelope l2.

The target 18 shown in FIG. 2 is photoconducting and comprises a single crystal wafer 19 having three regions 20, 22, 24 of different conductivity types and a semi-insulating coating 26 on the wafer 19. The first region 20 of the wafer 19 is that nearest the faceplate l4 and is N type The second region 22 is in the bulk ofthe wafer 19 and is N-type The third region 24 is a very thin surface state region 24 in the wafer 19 surface which is opposite the N"-type region 20. The surface state region 24 has a density of surface states such that this region 24 behaves like a P-type layer. For simplicity the three regions 20, 22, 24 of the wafer 19 will hereinafter be referred to as the N region" 20, the N-region 22 and the P-region" 24 respectively. The semi-insulating material is the thin coating 26 on the P-region 24 surface of the wafer 19.

Mode Of Operation In operation of the tube 10, the voltages applied to its various elements may be on the order of those voltages used in vidicon-type tubes known to those skilled in the art. The scanning means 16, which may include an electron gun device with electrostatic deflection, scans the semi-insulating material 26 with an electron beam whose electrons have various energies, the average energy of which is on the order ofa fraction of an electron volt. The semi-insulating layer 26 ther malizes" these beam electrons by bringing them to energy states which are described by a Fermi energy function at the temperature of the target 18 and collects enough electrons to bring the semi-insulating layer 26 to equilibrium potential. The N"-region 20 of the wafer 19 is a highly conductive accumulation region and is impressed with a potential a few volts positive with respect to equilibrium potential. In the absence oflight, the equilibrium potential will remain on the semi-insulating layer 26 and on the portion of the P-region 24 near the wafer surface for some time because the very thin Pregion 24, where it joins the N-region 22, forms a depletion region which acts as a potential barrier to the passage of electrons through it in the direction of the N -region 20. This is due largely to the fact that, in the absence of light, the depletion region has a rather low carrier concentration and is highly resistive to electrons with energies near the Fermi level.

When light travels through the faceplate 14 and into the N- region 22 of the wafer 19, it initiates the formation of electron-hole carrier pairs there. The electrons of these pairs travel toward the N -region 20 while the holes travel to the P- region 24. Thus in effect there is a conduction of negative charge from the P-region 24 and the semi-insulating layer 26 to the N"-region 20. The extent of such conduction is proportional to the amount of light falling on the target 18. It is for this reason that the target is referred to as being photoconductlve.

Upon return of the electron beam to an area on the semi-insulating layer 26 that has lost its equilibrium potential, the area is brought back abruptly to near equilibrium potential by the beam electrons. The abrupt change in potential of that area results in a corresponding current fluctuation in the con ducting N*-region 20. Fluctuations of current in the N -region 20 are taken as the signal output of the tube by a suitable electrical contact 28 to the N -region 20.

Materials And Fabrication The wafer 19 may be of any semiconducting material which has a suitable band gap and satisfies other well-known requirements ofa vidicon-type target, such as resistivity and degree of transparency. We prefer to use a single crystal comprised of a plurality of epitaxial layers of very high purity gallium arsenide phosphide (GaAs P whose composition has been regulated to make it responsive to visible light. For instance, a wafer 19 of GaAs P,, where x =0.45, would be suitable. Materials having a band gap of about 1.8 ev. are especially suitable for visible light television applications.

Fabrication of the wafer 19 may be accomplished by using well-known techniques such as liquid phase or vapor phase growth. In using methods such as vapor phase growth, a GaAs wafer on the order of 5 mil. thickness is used as a substrate for growth of the GaAs,,,P, wafer 19. It is preferred that the GaAs wafer be a single crystal. for in the epitaxial growth process the crystal structure of the substrate is continued in the growing crystal. However, it is not necessary that the substrate be a single crystal, as a target which is of several crystals, each continuous through the thickness of the wafer 19 would be acceptable, especially if the boundaries of the crystals are not resolvable. Thus, one could make a large area target out of several smaller area crystals. We prefer to use a discoid wafer but of course the wafer 19 can be any one of a variety of flat shapes. The reason for using a discoid is simply that it is more adaptable to vidicon tubes. Another purpose of using a GaAs substrate is that, since the GaAs R wafer 19 after the growth process is very thin, it is desirable to have a supporting structure under it to prevent breakage. Nor is it essential that the substrate be GaAs, for many substrate crystals having the right crystal structure can be used. For instance, a sapphire crystal could be used as a substrate.

During the growth of the GaAs P, the doping levels can be varied in such a way as to make the resulting wafer 19 a multilayered epitaxial structure. Nearest the GaAs surface the GaAs PBx has surface states, for this occurs naturally when one forms a crystal of GaAs P, with a light N-type doping level in the bulk central portion. The fact of the existence of these surface states is well known to those skilled in the art. The N-doping in the bulk central portion of the GaAs P should be light enough for the depletion region to extend from the P-region 24 throughout most of the N-region 22. We prefer to use silicon dopant to a level on the order of donors/cm. in the bulk for a resistivity there which will give optimum characteristics for operation in a vidicon.

The N -region of the GaAs R wafer 19 is on the order of a fraction of a micron thick and is farthest from the GaAs substrate. This portion is made N by, for instance, either adding the dopant, which may be silicon, during the growth of the wafer 18 or by diffusion after growing the wafer 18. N type doping should be heavy enough to give a high conductivity to the N-region 20 so that it is suitable as a signal plate.

The thickness of the wafer 19 should be determined to keep the capacitive lag to a desirable level and yet to have enough thickness for the three regions 20, 22, 24 of conductivity to be defined in the wafer 19. For instance, the thickness can be on the order of from 3 microns to 10 microns, although we prefer to use a thickness of about 5 microns.

After the GaAs P, wafer 19 has been grown on the GaAs substrate, the N,-region 20 of the GaAs P, may be attached to the inside surface of the faceplate 14 such as by adhering the two surfaces with transparent epoxycement. The attachment of the N*-surface 20 to the faceplate 14 surface lends sufficient support to the GaAs ,P, wafer 19 that the GaAs substrate may now be removed, such as by polishing or by preferential etching, thereby exposing the GaAs P, surface. lt is because the gallium arsenide is removed in this step that it is advantageous to make the substrate just thick enough to lend the necessary support, so that it can be readily removed.

The semi-insulating layer 26 is deposited on the exposed P region 24 surface which remains after the GaAs substrate has been removed from the wafer 19. The word semi-insulating" is used to indicate a material having a resistivity on the order of 10" or 10 ohm-cm. Many materials naturally have a resistivity in this range and some other materials may be adjusted by proper doping to have such a resistivity. Several materials such as resistive glasses can be used to form this layer 26. We prefer to use antimony trisulfide which has a resistivity of about 10 ohm-cm, is easily obtainable and can be readily evaporated to a P-type GaAs P surface.

It should be pointed out that although antimony trisulfide is commonly used as a photoconductor in vidicon tubes, its function in the present invention is not as a photoconductor, nor is it intended to function in a mechanical manner on the photoconducting wafer. The function of the semi-insulating layer 26 is rather to decrease dark current by thermalizing beam electrons as they impinge on that layer 26 so that electrons from the beam will not directly travel through the N-region 22 of the wafer 19 to the signal electrode N"-region 20. The average thermalized beam electron does not have sufficient energy to be injected from the P-region 24 into the N-region 22. Consequently, the narrow P-region 24 at the surface of the wafer 19, having a very low density of charge carriers, acts as a barrier region for electrons in the semi-insulating layer 26 and in the P-region 24. Thus, a barrier layer target 18 is formed using the natural surface states of the P-region 24 without the necessity of a diffusion, an array of islands or other complex structures on the scanned side of the target 18.

We claim:

1. A target for a vidicon-type camera tube having an envelope at one end of which is a transparent faceplate and having means for directing an electron beam toward said target, said target comprising:

a. a thin photoconductive single crystal wafer of wide bandgap gallium arsenide phosphide semiconductor material the bulk of which is lightly doped N-conductivity type, said wafer having a thin continuous surface state region of P-conductivity type near and including at least a portion of a major surface of said wafer scanned by said electron beam; and

b. a thin coating of semi-insulating material on the P-conductivity-type scanned surface of said wafer,

c. said semi-insulating layer having a resistivity on the order of from about 10 ohm-cm to about 10 ohm-cm and acting to thermalize electrons impinging on it from the electron beam to prevent those electrons from traveling through said wafer as dark current.

2. A target as claimed in claim 1 and wherein said wafer consists essentially of gallium arsenide phosphide semiconductor alloy having a donor density of about 10 donors per cubic centimeter.

3. A target as claimed in claim 1 and wherein said wafer consists essentially of gallium arsenide phosphide semiconductor alloy having a band gap between one and 3 electron volts.

4. A target as claimed in claim 1 and wherein said semi-insulating material is antimony trisulfide.

5. A target for a vidicon-type camera tube having a thin target, a first face of which is toward a scanning electron beam while a second face has light incident on it, said target comprising:

a. an epitaxial single crystal wafer of gallium arsenide phosphide semiconductor material having:

1. a first continuous region, including at least a portion of said second face of said target and extending through a portion of the thickness of said wafer, said first region being heavily doped N*to be electrically conducting; and

2. a second continuous region, including at least a portion of said first face of said target, said second region being doped lightly N-type and exhibiting P-type surface state properties at said first face of said target; and

b. a thin layer of semi-insulating material on said first face of said target toward said scanning beam,

c. said semi-insulating layer acting to thermalize electrons impinging on it when scanned by said electron beam and preventing said electrons from traveling through said second region to said first region as dark current.

6. A target as claimed in claim 5 and in which said crystal wafer is of a material having a band gap of about 1.8 electron volts.

7. A target as claimed in claim 5 and in which said crystal wafer is of gallium arsenide phosphide semiconductor alloy which has a band gap of about L8 electron volts.

8. A target as claimed in claim 5 and in which said semi-insulating layer is of antimony trisulfide.

9. A semiconductor vidicon-type camera tube target comprising a single crystal target electrode of N-conductivity-type gallium arsenide phosphide, said target electrode having at its scanned face a continuous P-conductivity-type surface state region and having on said scanned surface a thin layer of semiinsulating material with a resistivity of between about 10 ohm-cm and 10 ohm-cm which acts to thermalize electrons impinging on said face and to prevent those electrons from traveling through said target as dark current.

10. A target as claimed in claim 9 and in which said target electrode is of material having a band gap of about 1.8 electron volts.

ll. A target as claimed in claim 9 and in which said semi-insulating layer is of antimony trisulfide.

12. A target for a vidicon-type camera tube comprising:

a. a photoconductive single crystal wafer of gallium arsenide phosphide semiconductor material, being lightly doped N-conductivity type in its bulk to a doping level of less than 10 donors/cm and being from about 3 microns to about 10 microns thick, said wafer having,

I. a continuous N*-conductivity-type region signal plate at one major surface, and

2. continuous P-conductivity-type region at the other major surface scanned by an electron beam, that region being P-conductivity type because of the presence of surface states in the region; and

b. a layer of semi-insulating material on the P-conductivitytype region surface, said layer having a thickness of less than about 2 microns and a resistivity on the order of from about 10 ohm-cm to about 10 ohm-cm.

13. A target for a vidicon-type camera tube as claimed in claim 12 and wherein said wafer is of gallium arsenide phosphide material having a bandgap of from about 1.6 electron volts to about 2 electron volts.

14. A target for a vidicon type camera tube as claimed in claim 12 and wherein said wafer is doped in its bulk to a level of from about l0 donors/cm to about 10" donors/cm.

15. A target for a vidicon-type camera tube as claimed in claim 12 and wherein said layer of semi-insulating material is a layer of antimony trisulfide having a thickness on the order of 1 micron.

UNITED STATES PA'IEN'I ()FFKiF CERTIFICATE OF CORRECTION Patent No. 3 a 5 85 430 Dated June 15 19 71 n fl Ralph E. Simon and Robert L, Rodgers It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In line 1 of the Abstract and Column 3, lines 8, 10, 17, 29, 35, 39, 42, 48, 62, 63, 67, 69, Column 4, line 9, the formula should read as follows:

Column 3, line 63, "N should read N Signed and sealed this 16th day of November 1971.

(SEAL) Attest:

EDWARD M.F'LETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Acting Commissioner of Patents FORM O-1050 (10-69] USCOMM-DC mmnum U s hUViNNMFNI PIHNIIM. Ulill! N00 0 In Lu 

2. A target as claimed in claim 1 and wherein said wafer consists essentially of gallium arsenide phosphide semiconductor alloy having a donor density of about 1015 donors per cubic centimeter.
 2. continuous P-conductivity-type region at the other major surface scanned by an electron beam, that region being P-conductivity type because of the presence of surface states in the region; and b. a layer of semi-insulating material on the P-conductivity-type region surface, said layer having a thickness of less than about 2 microns and a resistivity on the order of from about 108 ohm-cm to about 109 ohm-cm.
 2. a second continuous region, including at least a portion of said first face of said target, said second region being doped lightly N-type and exhibiting P-type surface state properties at said first face of said target; and b. a thin layer of semi-insulating material on said first face of said target toward said scanning beam, c. said semi-insulating layer acting to thermalize electrons impinging on it when scanned by said electron beam and preventing said electrons from traveling through said second region to said first region as dark current.
 3. A target as claimed in claim 1 and wherein said wafer consists essentially of gallium arsenide phosphide semiconductor alloy having a band gap between one and 3 electron volts.
 4. A target as claimed in claim 1 and wherein said semi-insulating material is antimony trisulfide.
 5. A target for a vidicon-type camera tube having a thin target, a first face of which is toward a scanning electron beam while a second face has light incident on it, said target comprising: a. an epitaxial single crystal wafer of gallium arsenide phosphide semiconductor material having:
 6. A target as claimed in claim 5 and in which said crystal wafer is of a material having a band gap of about 1.8 electron volts.
 7. A target as claimed in claim 5 and in which said crystal wafer is of gallium arsenide phosphide semiconductor alloy which has a band gap of about 1.8 electron volts.
 8. A target as claimed in claim 5 and in which said semi-insulating layer is of antimony trisulfide.
 9. A semiconductor vidicon-type camera tube target comprising a single crystal target electrode of N-conductivity-type gallium arsenide phosphide, said target electrode having at its scanned face a continuous P-conductivity-type surface state region and having on said scanned surface a thin layer of semi-insulating material with a resistivity of between about 108 ohm-cm and 109 ohm-cm which acts to thermalize electrons impinging on said face and to prevent those electrons from traveling through said target as dark current.
 10. A target as claimed in claim 9 and in which said target electrode is of material having a band gap of about 1.8 electron volts.
 11. A target as claimed in claim 9 and in which said semi-insulating layer is of antimony trisulfide.
 12. A target for a vidicon-type camera tube comprising: a. a photoconductive single crystal wafer of gallium arsenide phosphide semiconductor material, being lightly doped N-conductivity type in its bulk to a doping level of less than 1015 donors/cm3 and being from about 3 microns to about 10 microns thick, saId wafer having,
 13. A target for a vidicon-type camera tube as claimed in claim 12 and wherein said wafer is of gallium arsenide phosphide material having a bandgap of from about 1.6 electron volts to about 2 electron volts.
 14. A target for a vidicon type camera tube as claimed in claim 12 and wherein said wafer is doped in its bulk to a level of from about 1014 donors/cm3 to about 1015 donors/cm3.
 15. A target for a vidicon-type camera tube as claimed in claim 12 and wherein said layer of semi-insulating material is a layer of antimony trisulfide having a thickness on the order of 1 micron. 